Ahh. Connectors. We love them and we hate them.
When you wire a robot there are many places where it can be tempting to take shortcuts to save time and money. Unfortunately connectors often suffer and lose that battle. From experience and many field tests I can not stress the importance of choosing proper connectors and having them located in the proper location. I will be focusing on stand alone connectors and not ones that are integrated into devices or into the power distribution system (terminal blocks, etc..) (that is for a future post).
I tend to think of connectors in two different categories. The first is for terminal devices, such as USB, ethernet (RJ-45), serial ports, hard drives, and antennas. These connectors are often dictated by the function (even though this is not always true). The second type is for transferring “stuff” (power and signals) from one part of the robot to another. In this second category are things like power and control signals.
I have three golden rules of connectors:
1. “The side with power should have sockets so you can not touch/short them. The side with no power should have pins since touching/shorting them should be fine if something has no power” – remember to keep your wiring safe. I have a co-worker who refers to power on the pins as “suicide plugs”
2. “If someone (or you) can plug it in wrong, they will” Trust me! – It is very easy to plug things in wrong, especially when someone else is tired and fumbling around with it in the dark.
3. “Use the proper crimp tool” – If not your wires will fall out when it is most important that they do not (part of Murphy’s law). I know proper crimp tools are expensive but if you want a reliable robot then it is necessary. Also having the tool and not using it properly is equal to not having the tool (just cheaper to remedy).
As a side note many of the brands will offer pins and sockets that can be either crimped or soldered. I prefer the crimp versions since they are easier to do properly and repeatedly, easy to rework, and the crimp grabs both the conductor and the insulation minimizing the chance of breaking the conductor. You should also be careful since several companies might make a connector that looks identical but the pins and sockets might not be compatible.
Where to put connectors?
Before you choose your connectors it is important to identify where you want to put the connectors. The general rule is to put connectors everyplace that you might EVER need to access, and not only where you plan on accessing.
For example if devices are located in parts of the robot that can separate (wheel arms, a head, etc..) there should be a connector at that point in case you ever choose/need to separate the components.
I also like to put connectors at the end of each motor/encoder lead instead of just extending the cables. This helps if the motor needs to be removed or swapped for another one.
What to look for in a connector?
When you are choosing a connector you want to choose one that fulfills your basic requirements such as size, number of conductors, ease of use (can you use operate the connector with a glove on), current capacity, maximum voltage, and price.
You will also want to consider things such as weather resistance, waterproof, strength, locking, ability to key the pins, idiot-proof connections, and ease of disassembly.
Connectors that I like
We now reach the main part of this article, the connectors. Here are a few series of connectors that I like (and some that I might not like).
I am going to start with what is probably my most used type of connector, the TE Connectivity CPC (circular plastic connector). These are circular connectors capable of holding up to 63 conductors. The pins/sockets are rated for 28AWG – 8AWG wire. I like this series since you get the rotating locking mechanism similar to mil-spec metal connectors for a reduced cost and weight.
To properly make a connection there are several components that you need to purchase:
Receptacle (a & b) – This is that side that has the pins that you will connect to. You can get Receptacles with a flange for mounting on a panel. There are also reverse sec connectors that have sockets in the receptacle, these are good for cases where power is in the receptacle and not coming from the plug side.
Plug (c) – This is the side that has the sockets that will connect to the receptacle.
Pins (d) – for the receptacle (assuming standard gender)
Sockets (e) – for the plug (assuming standard gender)
Backshell/Strain Relief – There are several types of strain relief options you can get to put on the back of the connector so that when people yank on the cable it is not pulled from the connector. These also help seal dirt from getting into the connector. In certain cases where you are tight for space you can leave these off.
Cover – This goes on the front mating surface of a receptacle or plug to help prevent damage or contact with the mating surface.
Keying Plugs – While I highly recommend using different connectors where things can get confused and plugged into the wrong place it is not always possible to do this. For these cases it is good to choose a connector with some extra conductor holes and uses keying plugs to make connectors not be mateable with ones that it should not mate with.
A good source of these connectors (and the source of these images) is mouser. While many distributors sell these connectors the best way to them is by going to the online catalog on mousers website and search for the connectors. They have good pictures that show the different connectors available and the options.
A similar type is the AMPHENOL Circular Mil Spec connectors. These are similar to the CPC connectors above but are made from metal. Since they are made from metal they are stronger, more resistant to wear and tear, and cost more.
The Deutsch line of connectors are very good and are commonly used in cars and heavy vehicles. There are many to choose from with different current capacity and conductor counts. The Deutsch connectors are easier to assemble and more resistant to water and dust then the CPC connectors. Once the pins/sockets are inserted into the housing there is a little clip that gets inserted to strengthen and seal the connector. The primary reason I do not like them and use CPC’s instead is that they do not have any strain relief for the connectors. You need to provide some external way to strain relief them about 3 inches behind the end of the connector. For connectors that just hang in the air or sit in the robot this is not always feasible.
A good source for more information is from LADD and they have a connector selection guide that is worth looking at.
Anderson powerpole connectors are great connectors for high current devices such as batteries. These connectors can go up to 350A, however they have different sizes based on current needs. The connectors are each for one wire, however they can slide into each other to create a larger connector of the shape and configuration that you choose. The nice thing with these are the two ends are the same however because of how they are designed they will only connect in one direction so you do not mess up and plug them in wrong. There is a notch between the connectors when you slide them together that is designed for a plastic insert to keep them from sliding apart. In practice I find a zip tie to work just as well (if not better). For more information check out their website.
D-Sub and HD D-Sub
D-Sub connectors are the ones we are used to seeing for things like (old) printers, serial ports, and VGA monitor ports. While we are used to seeing DB9 (9 pins), DB15 (15 pins high density (HD)), and DB25 (25 pins) connectors there are many other configurations. Some of the configurations can have data pins as well as power pins within the same connector. There are also a wide range of D-sub connectors available such as extra slim versions and waterproof versions. As with some other connector types you can get solder buckets built onto the connector or connectors that accept crimped pins and sockets (I advocate for the crimped versions).
When you purchase D-Sub/HD-Sub connectors you can also purchase backshells that act as strain relief for the cable. Some extra thin or panel mount versions do not have a backshell option. You should also be careful since the pins/sockets do not always work in connectors from a different manufacturer, and the D-Sub and HD-sub use different size pins/sockets with a different crimp tool.
I am not a huge fan of them but a connector list would not be complete without listing Molex connectors. Molex makes a wide assortment of wire to wire connectors. The reason I do not like them is that the metal pins/contact are a bit exposed, they are the least protected from the elements from the connectors presented, there is no strain relief, the pins/sockets can separate from the shell with repeated use, and the pins bend easily. The good things that I will say about them are that they are low-cost and most styles are good at preventing people from plugging them in the wrong way, several of the styles have a locking tab that helps hold the connector together.
Ethernet, USB, RF
For random ethernet, USB, and RF connectors I usually head to L-com. There are a bunch of wire to wire connectors as well as some panel mount options. Many of these will have covers that you can put over the connector to seal them when not in use. There are also cable shrouds that are molded plastic that fit over your cables to seal them to the connectors. Here is a bunch of connectors for harsh environments.
There are many types of common RF connectors. With all of these you need to watch for the maximum power, the maximum frequencies it can carry, and the resistance of the cables/connectors (cables are often not labeled if they are 50 ohms or 75 ohms). Here are some of the more common RF connectors:
u.fl – This is one of the smaller connectors and is usually connected direct to a PCB. It has a very limited number of connection cycles.
SMA – This is another small connector that is on many commercial radios.
RP-SMA – This is the reverse polarity version of the SMA connector above (note the center socket instead of a pin). This is designed for consumer applications were “non-trained” end users can change the cables/antennas. This is common on home routers.
BNC – This is an easy to use type of connector that is not suitable for outdoor use. This is often found on electronics test gear (oscilloscopes, function generators, etc..).
TNC – This is a threaded version of the BNC above.
N – N connectors are very popular with antennas, and are nice, since they are both threaded and waterproof. They are also the largest physical size of these connectors here.
Spade (or Blade)
These are similar to the main image in this article (the main image is ring terminals). They are a generic, common and inexpensive connector. They are good for occasional mating as the wire can fall out with a lot of usage or if it get pulled hard. They typically connect a single wire to another wire. The female part of this connector is often used to connect to battery terminals. There are many cheap crimpers for this type of connector but make sure to get one that puts a dent in the middle of the terminal and not just smashes the two sides. If is also advisable to get the connectors that are insulated instead of having bare metal exposed.
There are several submersible connectors out there. There are also submersible kits that essentially have you put silicon all over your connector and then heat shrink over that (messy).
I do not have direct experience with these but here are two sources of submersible connectors.
The first is from Cooper Industries and the other one is from Sure Seal.